1. Field of the Invention
The present invention relates to a surface-shaped light irradiation device of a side light type which uses a dot light source.
2. Description of the Related Art
Conventionally, for the purpose of backlighting a liquid crystal display element, a light source is arranged in front of one end surface of a rectangular light guiding plate, and light emitted from this light source is introduced into this light guiding plate so as to be emitted from one plate surface of this light guiding plate. A surface-shaped light irradiation device of a side light type is widely used, which illuminates the liquid crystal display element with this light guiding plate oppositely arranged at the back of the liquid crystal display element. As a light source of this type of illumination device (light irradiation device), a dot light source such as a light emitting diode (hereinafter referred to as LED) or the like is employed in order to promote compactness and slimness of the liquid crystal display module including the backlight.
Such a surface-shaped light irradiation device of a side light type using a light emitting diode as a dot light source is disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. H8-313902.
It is required of the surface-shaped light irradiation device using the dot light source that the dot light source be accurately set at a predetermined position with respect to the light guiding plate without fail. The reason for this is as follows. Since the dot light source is a light source which emits light radiately, it has more difficulty in obtaining a surface-shaped irradiation light having an intensity which is uniform over an entire light emitting area defined on a light emitting surface of the light guiding plate, than a linear light source such as a cathode tube or the like which emits light linearly, and thus various optical structures for obtaining an irradiation light having a uniform intensity distribution are employed therefor. As examples of such optical structures, a concentric ditch having its center correspond to the position at which the dot light source is set is formed in the back surface (counter light emitting surface) of the light guiding plate, or a prism sheet in which likewise a concentric ditch is formed is arranged at the side of the front surface (light emitting surface) of the light guiding plate.
Since there are differences in thermal expansion coefficient and shrinkage ratio among the components constituting the surface-shaped light irradiation device using the dot light source, repetition of heating and cooling of this light irradiation device due to changes in atmospheric temperature and turning on/off of the device itself causes the dot light source to slide from the right set position or changes the setting state of the dot light source, resulting in unevenness in the intensity distribution of the surface-shaped irradiation light and degradation of the intensity.